The accident occurred when the speeding mini-truck in which they were travelling hit a roadside tree at Charatala area under Chapra police station, killing them on the spot.
Besides high speed of the mini-truck, it was raining at that time which made the road wet and might have affected visibility, police said.
The deceased were identified as Rathindranath Biswas (47), Biswanath Biswas (49), Panchanan Biswas (50), Prakash Biswas (45), Prasanta Biswas (43) and Susanta Biswas (46).
They were returning after cremation of a close relative in the temple town of Nabadwip.
Another family member and the driver of the mini-truck were seriously injured and admitted to Shaktinagar District hospital, police said.
The police seized the mini-truck and sent the bodies for post-mortem.